The invasion

you probably are more advance than he is
get a vison ward or whatever and try catching him low heath against blue or red

now you killed him it's time to invade the ennemy jungle and kill every monster

by the time you finished that, your jungle is probably full so go back and empty it

take care : the ennemy jungle might want to get his vengence

Gank time :D

The good position for a nice gank is when the ennemy has gone more than halfway through the lane so you should nock him back towards your allies with your Devastating Charge

If you're level 6 you can also engage with your Onslaught of Shadows , you can also ult to finish the ennemy if he is getting away

Finish them

now just do what you want , you can stay in the jungle to try and finish your build or you might try to teamfight to victory :D
That might not be the best way to play Hecarim , but i'm not a pro and i like playing him dis way so i hope you like it :D
Well that's all , have a nice time playing hecarim :D